feature: support single batch gtdbtk classification

[franciscozorrilla]

🩻 Single batch submission of GTDBTk-based taxonomic classification 🚀

Current GTDBTk rule is great when dealing with many metagenomes, but when dealing with many isolate samples it makes more sense to follow this subworkflow task.

🐍 Snakefile tasks

• [x] create GTDBTk_singlerun for single job submission of small number of genomes

🔨 Additional modifications for full integration/support

• [ ] metaGEM.sh: add options + support for new tasks, helpfile message
• [ ] main readme: update helpfile message